Stringifier drops left/right shift precedences

>>> from pymbolic import parse
>>> parse("(a << b) >> 2")
RightShift(LeftShift(Variable('a'), Variable('b')), 2)
>>> str(parse("(a << b) >> 2"))
'a << b >> 2'
>>> parse("a << (b >> 2)")
LeftShift(Variable('a'), RightShift(Variable('b'), 2))
>>> str(parse("a << (b >> 2)"))
'a << b >> 2'